The present application relates to an electrostatically deflectable micromechanical device or a device comprising an electrostatically operating actuator for deflecting the mechanical device by means of a principle similar to the bimorph deflection principle.
Micromechanical devices comprising deflectable elements, such as, for example, beams or cantilevers, or plates or membranes, are employed in a plurality of technical fields, such as, for example, for beam deflection in projectors or SLM (Spatial Light Modulators), only to name a single representative example.
The electrostatic attractive force is used primarily as the drive principle for deflection in micro or nano actuators. The deflectable element and a fixed element are provided with a differing potential so as to cause a deflection by the electrostatic attractive force between same. The force here is indirectly proportional to the square of the distance between the deflectable element functioning as the first electrode and the fixed electrode. Due to this connection, the result, when a certain potential difference is exceeded, is a so-called “pull-in effect” where the deflectable element is accelerated towards the fixed electrode and touches same. For this reason, when using an electrostatic drive by means of a rigid electrode, a sufficient distance between the deflectable element and the rigid electrode is to be kept in mind, which in turn necessitates using high voltages in order to provide a compensation for the fact that the electrostatic force will be smaller due to the increased distance. Voltages of 100 volts or more are nothing unusual, which in turn causes further problems.
Apart from that, there are bimorph deflectable structures. Thermo-mechanical bimorph structures, for example, make use of the differing expansion of different materials by locating heating structures so as to make use of the bimorph principle for deflection. However, the response times here are low, the temperature differences necessitated for big deflections are high and the selection of materials naturally is limited, all these being disadvantages for a realization thereof. Another bimorph principle uses piezoelectric or electrostrictive elements in order to deflect a deflectable element in a bimorph way. Handling the materials necessitated here is a problem, since these make integration into conventional semiconductor manufacturing processes impossible. All in all, producing such drive structures is complicated and expensive.
One approach for solving these problems is presented in WO2012095185A1. FIG. 21 shows a micromechanical device which is described there. A deflection similar to the bimorph principle here is achieved by generating, spaced apart from the neutral fiber of the deflectable element, a transverse contraction or transverse expansion in a lateral direction by means of the electrostatic attractive force, which in turn curves the deflectable element. FIG. 21 shows the micromechanical device in cross-section. The micromechanical device of FIG. 21 comprises a deflectable element 10 which is formed in the shape of a beam or a membrane. The cross-section here is taken in the direction of thickness. In particular, the deflectable element 10 comprises a beam or membrane 20 through which the neutral fiber 16 of the deflectable element 10 extends, wherein a plate capacitor 14 is formed on one side of the beam or membrane 20 that is spaced apart from the neutral fiber 16. A proximal electrode of the capacitor 14 is formed by the beam or the membrane 20 itself. A distal electrode 18 of the plate capacitor 14 is opposite the beam or membrane 20. Thus, the plate capacitor 14 is divided, in a lateral direction 12, into segments 22 between which the distal electrode 18 and the proximal electrode 20 are connected mechanically between the segments 22 at segment boundaries 24. Both electrodes 20 and 18 are implemented to be laterally continuous. When providing the plate capacitor 14 with a voltage, the electrostatic attractive force between the electrodes 18 and 20 causes a transverse expansion which causes the deflectable element 10 to be deflected along the direction 12 such that the neutral fiber 16 is curved stronger than the plate capacitor 14. Compared to the electrostatic actuators having been described above, a principle similar to the bimorph principle is made use of in accordance with FIG. 21. The electrodes 20 and 18 are essentially rigid. Expressed differently, no bending of one or both electrodes 18, 20 per segment is to be achieved, which bends would then be passed on from segment to segment, but the expansion in the distance of the capacitor 14 from the neutral fiber 16 causes curving in accordance with the bimorph principle. This means that problems, such as, for example, the pull-in effect, are not to be expected. This in turn allows the distance between the electrodes 18 and 20 to be implemented to be enormously small and, consequently, connected thereto the electrostatic attractive forces to be enormous in order to generate a sufficient transverse expansion. What is more, due to the principle of deflection discussed above, which is similar to the bimorph principle, the deflectable element 10 may be and is to be bend-proof. For this reason, transmission of forces and high stability to externally caused movements are possible using the micromechanical device of FIG. 21.
However, a problem when manufacturing the micromechanical devices presented in the above WO document is realizing the plate capacitor 14. As is shown in FIG. 21, the electrodes 18 and 20 are electrically insulated from each other by portions 26 of an insulation layer which are arranged at the segment boundaries 24 and provide for both the mutual electrical insulation and the mechanical connection mentioned before. In other words, the portions 26 form insulating spacers which are arranged vertically or in a direction of thickness between the electrodes 18 and 20. The structuring of this insulation layer, which in FIG. 21 is illustrated in a hatched manner, causes problems when manufacturing the micromechanical device of FIG. 21. While the top electrode 18 and that side of the beam or membrane 20 facing the top electrode 18 may comprise a topography in the micrometer scale, the mutually facing surface thereof is, as regards roughness, definitely to be smooth, that is on the nanometer scale, since deviations from this smoothness result in electrical voltage peaks and breakthroughs, that is limit the voltage applicable and, thus, the lifetime or deflection achievable. However, achieving smoothness is a problem for the following reasons. When manufacturing, the gaps 28 between the spacers 26 or voids 28 of the segments 22 between the electrodes 18 and 20 are filled with a sacrificial material which has to be removed again after applying the electrode 18. The material from which the spacers 26 are made, thus needs to be inert relative to etching of the sacrificial material in these gaps 28, since otherwise the insulation islands or spacers 26 would be attacked or even removed when etching the sacrificial layer. The structuring of the insulation islands 26 causes the greatest difficulty when manufacturing the micromechanical device in accordance with claim 21. The material of the insulation island may, for example, be silicon oxide, silicon nitride or aluminum oxide. Such materials are mainly structured using dry etching, such as, for example, reactive ion etching (RIE). Wet etching by means of an etching liquid would be possible in principle, but provides a poorer lateral resolution, since etching is isotropic, which means that the mask here is under-etched. Thus, dry etching is an advantageous type of structuring for the insulation layer between the electrodes 18 and 20. In order to be able to check on the dry etching in the depth well, a so-called “etch stop layer” which is to be placed underneath the layer to be etched, that is here the sacrificial material at the positions 28, is usually necessitated. As is shown in FIG. 21, the bottom electrode 20, that is the beam or membrane 20, is located here at these positions 28. However, the bottom electrode 20 is poorly suitable as an etch stop layer for structuring the insulation layer, since dry etching tends to etch on the bottom electrodes in a more than insignificant manner, which in turn increases the roughness of that side of the bending beam or membrane 20 facing the top electrode 18. As has already been mentioned, the bottom electrode 20 or beam/membrane 20 is to be smooth on the nanometer scale on that surface facing the top electrode 18. This means that it is not possible easily to structure the insulation islands 26 using dry etching and an etch stop on the bottom electrode 20 such that the surface of the bottom electrode 20, after etching, exhibits the desired very small roughness. Additionally, in order to achieve a maximum deflection effect, a topography of the bottom and top electrodes 18, 20 of the plate capacitor 14 is desired, such as, for example, a V-shaped cross-section, segment by segment, of the gap 28 between the electrodes 18 and 20 with a corresponding V-shaped topography of that side of the beam or membrane 20 facing the top electrode 18. However, such topographies are difficult to manufacture. If a desired topography on that side of the beam or membrane 20 facing the top electrode 18 is produced, the following technological steps are influenced strongly by this topography and, in dependence of this topography, are to be optimized differently than would be the case with a completely planar implementation of the plate capacitor 14, as is illustrated in FIG. 21 for reasons of simplicity. Structuring the insulation layer so as to generate the insulation islands 26 is, in particular, among these subsequent steps. However, this deteriorates the situation when generating the insulation islands further.
Consequently, the object of the present invention is providing a micromechanical device comprising an electrostatic deflection principle and the advantages of a bimorph deflection or a deflection similar to the bimorph deflection, which additionally may be manufactured more easily and/or exhibit an improved operating behavior.